The present invention is directed to an adiabatic fluidized bed reactor for combusting non-uniform particulate matter and, more specifically, to an adiabatic fluidized bed reactor in which pressurized air is supplied into a bed of granular material through openings in a support surface for the bed and through openings in the reactor walls, and to a method of operating an adiabatic fluidized bed reactor.
In fluidized bed reactors for combusting particulate material, the material to be combusted is fed over a bed of granular material, usually sand. In such reactors, it is desirable to be able to vary the amount of particulate material fed to the reactor and, concomitantly, the amount of pressurized air supplied to the reactor over as wide a range as possible. The hydrodynamic turndown ratio of a reactor, which is defined as the ratio of pressurized air flow at maximum reactor load to pressurized air flow at minimum reactor load, is a measure of the ability of a reactor to operate over the extremes of its load ranges.
In conventional fluidized bed reactors, pressurized air is fed to the reactor bed through air distribution nozzles located in a grate that supports the bed of granular material. An example of such a conventional fluidized bed is disclosed in U.S. Pat. No. 4,075,953 to Sowards, specifically in the embodiment depicted in FIG. 1 of that patent, and in U.S. Pat. No. 3,907,674 to Roberts et al. At conventional bed heights, the minimum pressure drop across the air distribution nozzles needed to maintain proper fluidization at minimum reactor load ranges from 2 to 4 inches of water. The former figure relates to medium height beds while the latter refers to deep fluidized beds. At maximum reactor load, pressure drops exceeding 8 to 12 inches of water are not economically practical inasmuch as excessive forced draft fan horsepower would be required. Since the pressure drop across the air distribution nozzles is directly proportional to the square of the volumetric air flow rate, it can be seen that the hydrodynamic turndown ratio in such conventional reactors is limited to 2 to 1.
In order to obtain hydrodynamic turndown ratios in excess of 2 to 1, the prior art reverted to the use of fluidized beds with multiple compartments, i.e., fluidized beds with multiple beds. Two conventional beds give a 4 to 1 turndown ratio and three beds give a 6 to 1 turndown ratio. A significant disadvantage of such multiple-bed reactors is that a cold, shut down bed requires time to be brought back in line. Moreover, although multiple compartments can be used for square or rectangular-shaped fluidized beds, the difficulties encountered when using multiple compartments with cylindrical-shaped reactors outweigh the advantages. Thus, as can be seen from the above discussion, notwithstanding the need for a fluidized bed reactor with turndown ratios in excess of 2 to 1, the prior art has not satisfactorily provided a solution.
In the previously discussed conventional fluidized bed combustors, fluidizing air is supplied to the bed only from air distribution nozzles located in the bed support. In the past, air has also been supplied through the reactor walls of a fluidized bed reactor for the purpose of pneumatically feeding particulate material into the bed, such as disclosed in U.S. Pat. No. 3,897,739 to Goldbach. Additionally, air has been supplied through the reactor walls of a fluidized bed combustor above the surface of the bed for the purpose of effecting mixing and complete combustion, such as disclosed in U.S. Pat. No. 3,863,577 to Steever et al. Finally, in non-adiabatic fluidized bed combustors that utilize internal cooling surfaces and that do not use a bed of granular material when solid carbonaceous particulate material is burned, air has been supplied through the reactor walls of the combustor for the purpose or promoting combustion, such as disclosed in U.S. Pat. No. 4,165,717 to Reh et al. As can be seen, the prior art has not supplied pressurized air through the reactor walls of an adiabatic fluidized bed reactor for the purpose of increasing the turndown ratio of the reactor, nor has the prior art provided a method or structure for obtaining hydrodynamic turndown ratios in excess of 2 to 1 when the fluidized bed reactor utilizes a single bed of granular material.